1. Field of the Invention
This invention relates to a disc drive apparatus and, more particularly, to improvement in the tracking control function of the disc drive apparatus and to automation of seek time adjustment.
2. Prior Art
With the conventional disc drive apparatus, the magnetic disc or disc medium is driven into rotation by a spindle motor, and a data-recording tracks or data tracks are formed concentrically or spirally on the recording layer of the magnetic disc surface. The following methods have been known and used for forming the data tracks on the magnetic disc.
For example, for forming the data tracks on the magnetic disc surface, there are formed servo tracks, each having a servo pattern, referred to as a tri-bit servo pattern, such as is proposed in the U.S. Pat. No. 3,691,543, on the inner most and the outermost peripheries of the disc. Using a head position sensor, the magnetic head is controlled to be at a predetermined position in the vicinity of, for example, the innermost servo track. A tracking error signal, detected upon reproducing the servo track, is used to shift the magnetic head to a center of the servo track, that is a servo track center, so that the tracking error signal will be equal to zero, and a detection error of the head position sensor, obtained at this time from the head position sensor, is found. The similar operation is performed on the outermost servo track to find the detection error of the head position sensor at the outermost periphery of the magnetic disc.
Then, during the time when the data track or tracks are sequentially formed in a data area delimited between the two servo tracks, the magnetic head is controlled to be positioned in the vicinity of a desired data track center with the aid of the head position sensor. Then, as a function of the position of the data track scanned by the magnetic head, a required correction amount, referred to hereinafter as an off-track correction amount, is found by, for example, linear approximation, with the aid of two detection errors of the head position sensor, and tracking control of the magnetic head is performed on the basis of the off-track correction amount. In this manner, a number of data tracks are formed concentrically with respect to the servo tracks.
For recording or reproducing data on or from each data track, detection errors of the head position detection sensor are detected at a predetermined time interval, and the off-track correction amount at a desired data track is found on the basis of the detection error. Tracking control of the magnetic head is performed on the basis of this off-track correction amount.
In this manner, the magnetic head may be tracking-controlled on the basis of the servo tracks formed on the innermost and outermost peripheries of the magnetic disc.
On the other hand, with a disc drive apparatus in which the data track pitch of the magnetic disc is reduced to increase the recording capacity, the following system is used as the head position sensor.
The light emanating from a light source is passed through a reticle by way of a scale mounted on an arm of a magnetic head and the light thus transmitted through the reticle is detected by photo-electric converter segments. The scale and the reticle are provided with slits at a predetermined interval and when the position of the scale is changed relative to that of the reticle with movement of the magnetic head, the volume of the light incident on the photoelectric converter segments by means of the scale and the reticle is changed. For example, in the case of the head position sensor comprised of four juxtaposed photoelectric converter segments, four-phase detection signals DET.sub.1 to DET.sub.4 having different phases are produced from the respective photoelectric converting segments, as shown in FIG. 1. The positions T1 to T4, corresponding to the phases of these detection signals crossing the zero level in, for example, the rightwardly rising direction, prove to be the track centers.
Hence, with the above head position sensor generating the four-phase detection signals, it is necessary to calculate the above mentioned off-track correction amount for each of the photoelectric converting segments. Therefore, servo tracks associated with the respective photoelectric converting segments are provided as the innermost and outermost tracks, that is a total of eight servo tracks, ar provided on the disc surface, for calculating the above mentioned off-track correction amounts.
On the other hand, with disc drive apparatus employing plural magnetic discs for enlarging the recording capacity, the following tracking control is used.
The magnetic discs are arranged coaxially and driven into rotation by a sole spindle motor. When the servo track is formed only one disc surface, and the data track is formed on the disc surface on which the servo track is formed, the magnetic head scanning the disc surface on which the servo track is formed and the magnetic head scanning the disc surface on which the servo track is not formed are kinematically interlocked so that data tracks may be formed simultaneously on the disc surface carrying the servo track and the disc surface devoid of the servo track. In this case, even if the magnetic heads suffer from different mounting errors, these magnetic heads are rotated in unison, so that by finding the off-track correction amount of the head position sensor with the use of the servo track formed on one disc surface, it becomes possible to perform a correct tracking control of the magnetic heads scanning the disc surfaces.
However, with the disc drive apparatus adapted for performing tracking control of the disc surface with the use of the servo track formed on only one disc surface, the problem is raised that sufficiently accurate tracking control cannot be achieved.
For example, the mounting accuracy may be changed by, for example, several micrometers (.mu.ms) between the magnetic heads scanning the disc surfaces, due to, for example, changes in the ambient temperature, so that, even if tracking control of the magnetic heads scanning the disc surfaces is performed unanimously on the basis of the servo track formed on one disc surface as described above, a tracking error corresponding to changes in the mounting accuracy between the magnetic heads due to changes in temperature is produced. As a result, the tracking control accuracy is correspondingly deteriorated to worsen the error rate.
Among the methods for coping with this problem, there is a method as disclosed in Japanese Patent Publication No. 220274/1989. Thus there is known a method comprising forming a servo track on each of the disc surfaces and correcting an off-track amount of the head position sensor on each disc surface.
However, a prolonged operation would be necessitated if a sum total of eight servo tracks associated with the respective phases of the detection signals of the head position sensor is formed on the innermost and outermost peripheries of the respective disc surfaces, and the off-track amounts of the head position sensors are corrected for each disc surface.
On the other hand, when recording or reproducing data on or from the magnetic disc, a tracking control is performed, according to which, after the magnetic head is moved at a high speed to the target data track (seeking), the magnetic head is caused to scan the center of the data track.
For performing such high-speed seeking of the magnetic head, the velocity of the movement of the head arm carrying the magnetic head is controlled as a function of the distance from the current head position to the target track, that is the number of tracks to be crossed by the head, for controlling the velocity of the movement of the magnetic head carried by the foremost part of the head arm along the radius of the disc.
Referring to FIG. 2, if the target track is at some distance, for example, the magnetic head is accelerated at a maximum acceleration up to a constant velocity, usually the maximum velocity, the magnetic head is then moved at the constant velocity until the number of the remaining tracks is reduced to a predetermined number, and the magnetic head is then decelerated at a predetermined deceleration after the number of the remaining tracks is lesser than a predetermined value. That is, while the velocity of the magnetic head is continuously detected, the magnetic head is decelerated to a velocity which is inversely proportionate to the number of remaining tracks, until the velocity is reduced to zero in the vicinity of the target track. This control mode is referred to hereinafter as the velocity control mode. Then, as the magnetic head position is continuously detected, the magnetic head is then position controlled so that the head is positioned at the center of the target track. This control mode is referred to hereinafter as the position control mode. In this case, the time which elapses since the magnetic head starts to be accelerated until the velocity of the magnetic head is reduced to zero is referred to as the seek time. Meanwhile, the time which elapses since the velocity of the magnetic head is reduced to zero until the magnetic head is positioned at the center of the target track is referred to as the setting time.
On the other hand, when the target track is at a not-distant position, the magnetic head is accelerated at the maximum acceleration, as shown in FIG. 3. When the number of the remaining tracks is lesser than a predetermined value, the magnetic head is decelerated at a predetermined deceleration. that is, as the velocity of the magnetic head is detected continuously, the magnetic head is controlled to be decelerated to a velocity which is inversely proportional to the number of the remaining tracks so that the velocity of the magnetic head is reduced to zero in the vicinity of the target track. then, as the magnetic head position is detected continuously, the magnetic head is shifted in controlled manner so that the magnetic head is positioned at the center of the target track.
It will be noted that the above mentioned seek time significantly influences the access time which is among the critical properties in recording or reproducing data in the disc drive apparatus, and hence standards or prescriptions are provided for the seek time. Thus, in the manufacture process of the disc drive apparatus, a sufficient adjustment operation is performed before shipment of the apparatus so that the seek time during which the magnetic head seeks one-thirds of the disc radius is included within a predetermined standard value.
The conventional practice has been to perform such seek time adjustment by a manual operation by using a volume knob. However, the weight of the head arm, the torque of the motor driving head arm or the output of the head position sensor are fluctuated from one disc apparatus to another, such that a number of adjustment steps are necessitated, thus increasing manufacture costs. On the other hand, the volume knob is inferior in reliability to the usual resistor.